Translation:I eat lunch.
As others have commented, the お here is an honorific and is not a necessary part of the word.
However, I have to disagree with all the comments about it indicating whose food is being referred to. In many cases, like お名前, the use of お does indicate respect for the listener (and hence the word's directionality), but there is a subset of keigo known as 美化語 (bikago = "beautification speech"). https://en.wikipedia.org/wiki/Honorific_speech_in_Japanese#Word_beautification
As it says in the article, sometimes Japanese people make their speech polite for politeness' sake because it sounds nicer, classier, or more "beautiful". In some cases (such as this sentence), the お (or ご) honorific is used withouth the speaker being intentionally deferential to the listener.
The most common case of this is お茶. You will almost never hear a Japanese person refer to tea, their own or anyone else's, as simply 茶 (unless it is part of another word like 抹茶 maccha or ウーロン茶 uuroncha). The word 茶 has been "beautified" so much in Japanese culture, that to say it without the お sounds crass or rough.
It seems to be speaking in more of a general sense rather than something that you will do in the future. Although in the right context, I think you could use it to mean "I will eat lunch" but for a more direct translation the "will" should be left out. I'm no native Japanese speaker though so I could be wrong
I think -- but I could be wrong -- that to say that you will eat "the lunch" you would need to topicalize it with a は, like in those "I don't eat X" sentences some lessons ago. A simple "I will eat lunch" (without the "the") should be correct, however. Duolingo can be a bit picky about your English, so whenever you get a surprising "Wrong answer!" the first thing to do should be to check if you slipped up in your English grammar. :)
The previous question was "bangohan wo tabemasu." to which the answer was "I will eat dinner." Now this question, using exactly the same structure, says "I eat lunch." is the correct answer (omitting the 'will' from the previous answer). I guess I still haven't grasped the nuances here, because "I will eat" and "I eat" have two rather different meanings in English. Does the Japanese language deal with past/present/future tense differently than English?
Yes, Japanese tenses don't always line up with English tenses. In Japanese, the ます form (and the equivalent う/る dictionary forms) of a verb is used in three different cases:
- 1: general, time-less sentences, e.g. "What goes up, must come down" (上がる物は必ず降ります【あがるものはかならずおります】)
- 2: habitual or routine actions, e.g. "I often eat lunch alone" (良く一人でお昼ごはんを食べます【よくひとりでおひるごはんをたべます】)
- 3: near-future actions, which are planned, or have a reasonable likelihood, to happen, e.g. "I will eat lunch with you today" (今日、一緒にお昼ごはんを食べます【きょう、いっしょにおひるごはんたべます】)
So, as you can see, cases 1 and 2 line up with English's simple present tense, but case 3 matches up with future tense (whether that be "I will" or "I am going to"). Which translation is "correct" depends largely on the context; sometimes context from elsewhere in the sentence is enough (as in my examples) but other times, you need context from the rest of the conversation.